SQL uses a
three-valued logic system with true, false, and null, which represents "unknown". Observe the following truth tables:

a

b

a AND b

a OR b

TRUE

TRUE

TRUE

TRUE

TRUE

FALSE

FALSE

TRUE

TRUE

NULL

NULL

TRUE

FALSE

FALSE

FALSE

FALSE

FALSE

NULL

FALSE

NULL

NULL

NULL

NULL

NULL

a

NOT a

TRUE

FALSE

FALSE

TRUE

NULL

NULL

The operators AND and OR are commutative, that is, you can switch the
left and right operand without affecting the result. But see
Section
4.2.14 for more information about the order of evaluation of
subexpressions.